1. Field of the Invention
The present invention relates to a pellicle for photolithography that can be used for preventing dust deposition in the manufacture of semiconductor devices, printed circuit boards, or liquid-crystal displays.
2. Description of the Related Art
In the manufacture of semiconductor devices such as LSIs or liquid-crystal displays, patterns are produced by irradiating a semiconductor wafer or a glass substrate for liquid crystals with light, but if dust particles deposit onto the photomasks or reticles (both are referred to hereinbelow simply as “photomasks”) that are used in this process, the dust particles will shield or deflect the light, thereby destroying the transferred pattern.
For this reason, the operations of producing a pattern are usually conducted in a clean room, but even there the photomasks are difficult to maintain in a clean state all the time. For this reason, a method of mounting a pellicle is undertaken for preventing dust from deposition onto the photomask surface.
In this case, foreign matter deposits on the pellicle, rather than depositing directly onto the photomask surface. Therefore, if the exposure light is focused to the photomask pattern during the photolithography, the foreign matter present on the pellicle membrane does not affect the light transmission.
A pellicle is usually fabricated by placing a transparent pellicle membrane composed of nitrocellulose, cellulose acetate, or a fluorocarbon resin with good light transmissivity on the upper end surface of a pellicle frame (referred to hereinbelow simply as “frame”) made of aluminum, stainless steel or polyethylene, applying a solvent, and bonding by blowing dry air (see Japanese Patent Application Laid-open No. 58-219023) or by bonding with an adhesive such as an acrylic resin or epoxy resin (see U.S. Pat. No. 4,861,402 or Japanese Examined patent Application No. 63-27707). Furthermore, a pressure-sensitive adhesive layer comprising a polybutene resin, polyvinyl acetate resin, or acrylic resin for attaching to a photomask and a separation layer (separator) for protecting the pressure-sensitive adhesive layer are provided on the lower end surface of the frame.
In order to avoid the difference in pressure between the inside space of a pellicle surrounded by the pellicle membrane bonded to the frame and the outside when the pellicle is attached to the photomask, a small air-escape opening for balancing the gas pressure is sometimes provided in a part of the frame and a filter is disposed on the small opening to prevent the intrusion of foreign matter from the air moving through the small opening (Japanese Examined Utility Model Application No. 63-39703).
The pellicle membrane is usually formed from a very thin resin film. Therefore, in order to attach the film to the frame, in a slack-free fashion, it has to be bonded to the frame in a state in which an adequate tensile force is uniformly applied in all the directions.
For this reason, with rectangular pellicles, which are usually used, a certain inward curving can be observed in the frame after the pellicle membrane has been bonded thereto due to the tension of the membrane. This phenomenon becomes significant when the frame has a large dimension, for example, as in the case of large pellicles used for the manufacture of printed circuit boards or liquid-crystal displays.
On the other hand, in order to reduce cost, the photomasks are required to ensure a large exposure area. The resultant problem is that, unless the inward curving of the frame is minimized, the exposure area (surface area) that can be used decreases accordingly. This problem obviously can be resolved by increasing the frame rigidity, for example, by increasing the cross-sectional area of the frame, but the above-described problem of exposure area is associated with the inside of the frame and a clearance for handling, which is required when the photomask is fixed or transported, has to be ensured on the outside of the frame. For this reason, a limitation is placed on the size of the cross-sectional area of the frame. Accordingly, each side of the frame is usually formed to have a straightly linear form determined by those limitations.
An attempt was made to resolve this problem, for example, with a method of obtaining an almost straightly linear form by using a tensile force of a membrane to curve the longer sides of a frame that were caused to bulge outward in advance (see Japanese Patent Application Laid-open No. 2001-42507).
The advantage of this method is that the fabrication can be easily conducted due to a relatively simple worked form and the desired exposure areas also can be ensured, but a problem is associated with dimensional accuracy. For example, as shown in FIG. 4A, when a longer side 41 of the frame is curved outward, if a pellicle membrane is attached to the frame, then the shape shown in FIG. 4B is assumed, curved zones remain in the portions 44 close to the corners, and the entire area surrounded by the frame cannot be used as the exposure area. Thus, the area that can be used as the exposure area is the shadowed portion 43 surrounded by straight lines.
The curved portions remaining close to the corners of the frame are caused due to the fact that a tensile force of the pellicle membrane and the curved form of the bulged frame are not properly balanced. Another problem associated with this method is that the pellicle is difficult to align when attaching to the photomask and positioning accuracy of the attached pellicle on the photomask decreases due to deformation of the outer form of the frame.
Furthermore, because no reference position is appropriately set for the curved side that bulged to the outside of the frame, accurate size measurements after processing are difficult to be conducted. The resultant problem is that accuracy control during frame manufacture is difficult and processing accuracy can easily be decreased.
With such a measure involving curving of the frame, if the side length is increased, the shorter side also has to be curved, but if a curved form is produced on the shorter side in addition to the longer side, there is absolutely no section that should serve as a reference in size measurements and accurate size measurements after processing are difficult to be conducted. As a result, dimensional accuracy decreases even more significantly.